1. Field of the Invention
The present invention relates generally to photovoltaic solar cells, and, more particularly, to a high-efficiency, multi-junction, lattice-matched, monolithic solar cell device. Even more particularly, the present invention is directed to a solar cell device that includes a semiconductor cell that is lattice-matched to GaAs and has an optimal band gap for enhancing the energy conversion efficiency of the solar cell device.
2. Description of the Prior Art
Solar energy is an important source of energy. Photovoltaic devices fabricated from layers of semiconductor materials, commonly called solar cells, are presently used to convert solar energy directly into electricity for many electrically powered applications. However, greater solar energy to electrical energy conversion efficiencies are still needed in solar cells to bring the cost per watt of electricity produced into line with the cost of generating electricity with fossil fuels and nuclear energy and to lower the cost of telecommunication satellites.
Solar energy, essentially light, comprises electromagnetic radiation in a whole spectrum of wavelengths, i.e., discrete particles or photons at various energy levels, ranging from higher energy ultraviolet with wavelengths less than 390 nm to lower energy near-infrared with wavelengths as long as 3000 nm. Between these ultraviolet and infrared wavelengths or electromagnetic radiation energy levels are the visible light spectrum, comprising violet, blue, green, yellow, orange, and red wavelengths or energy bands.
Because a semiconductor layer of a solar cell absorbs photons with energy greater than the band gap of the semiconductor layer, a low band gap semiconductor layer absorbs most of the photons in the received solar energy. However, useful electrical power produced by the solar cell is the product of the voltage and the current produced by the solar cell during conversion of the solar energy to electrical energy. Although a solar cell made from a low band gap material may generate a relatively large current, the voltage is often undesirably low for many implementations of solar cells.
To achieve the goal of using most of the photons in the solar spectrum while simultaneously achieving higher output voltage, multi-junction solar cells have been developed. Multi-junction solar cells generally include multiple, differently-configured semiconductor layers with two or more solar energy conversion junctions, each of which is designed to convert a different solar energy or wavelength band to electricity. Thus, solar energy in a wavelength band that is not absorbed and converted to electrical energy at one semiconductor junction may be captured and converted to electrical energy at another semiconductor junction in the solar cell that is designed for that particular wavelength range or energy band.
Several difficulties have arisen in producing these multi-junction solar cells which has limited their energy conversion efficiency. First, it has proven difficult to fabricate each semiconductor junction so as to maintain high photovoltaic device quality and simultaneously the appropriate band structure, electron energy levels, conduction, and absorption, that provide the photovoltaic effect within the solar cell as the multiple layers of different semiconductor materials are deposited to form the solar cells. It is understood that photovoltaic quality may be improved in monolithic solar cells by lattice matching adjacent layers of semiconductor materials in the solar cell, meaning that each crystalline semiconductor material that is deposited and grown to form the solar cell has similar crystal lattice constants or parameters. Mismatching at the semiconductor junctions in the solar cells continues in many fabricated solar cells to create defects or dislocations in the crystal lattice of the solar cell, which causes degradation of critical photovoltaic quality characteristics, such as open-circuit voltage, short-circuit current, and fill factor. Second, the energy conversion efficiency, including photocurrent and photovoltage, has proven difficult to maximize in multi-junction solar cells. Photocurrent flow can be improved if each solar cell junction of the semiconductor device can be current matched, in other words, to design each solar cell junction in the multi-junction device in a manner such that the electric current produced by each cell junction in the device is the same.
Current matching is important when a multi-junction solar cell device is fabricated with the individual semiconductor cells in the device connected (i.e., stacked) in series, because, in a series circuit, current flow is limited to the smallest current produced by any one of the individual cells in the device. Current matching can be controlled during fabrication by selecting and controlling the relative band gap energy absorption capabilities of the various semiconductor materials used to form the cell junctions and the thicknesses of each semiconductor cell in the multi-junction device. In contrast, the photovoltages produced by each semiconductor cell are additive, and preferably each semiconductor cell within a multi-cell solar cell is selected to provide small increments of power absorption (e.g., a series of gradually reducing band gap energies) to improve the total power, and specifically the voltage, output of the solar cell.
To address the above fabrication problems, a large number of materials and material compounds have been utilized in fabricating multi-junction, monolithic solar cell devices. However, these prior art solar cells have often resulted in lattice-mismatching, which may lead to photovoltaic quality degradation and reduced efficiency even for slight mismatching, such as less than 1 percent. Further, even when lattice-matching is achieved, these prior art solar cells often fail to obtain desired photovoltage outputs. This low efficiency is caused, at least in part, by the difficulty of lattice-matching each semiconductor cell to commonly used and preferred materials for the substrate, such as germanium (Ge) or gallium-arsenide (GaAs) substrates. As discussed above, it is preferable that each sequential junction absorb energy with a slightly smaller band gap to more efficiently convert the full spectrum of solar energy. In this regard, solar cells are stacked in descending order of band gap energy. However, the limited selection of known semiconductor materials, and corresponding band gaps, that have the same lattice constant as the above preferred substrate materials has continued to make it difficult to design and fabricate a multi-junction, monolithic solar cell that efficiently converts the received solar radiation to electricity. Therefore, a need remains to provide semiconductor materials with desirable band gap ranges and with a lattice constant that is substantially equivalent to that of Ge or GaAs to improve prior art photovoltage output and solar energy conversion efficiency for solar cells.